This invention relates to capacitive pressure transducers and, more particularly, to such a transducer which is capable of providing accurate measurements within high temperature environments, and a measuring circuit therefore which eliminates the effect of temperature and resistance changes on the measured output of such transducer.
Because of their ruggedness, small size and accuracy, diaphragm-type capacitance transducers, such as the one disclosed and claimed in my U.S. Pat. No. 3,027,769, are commonly used in many different environments. However, their use in high temperature and/or humid environments has been limited. More particularly, the resistance of certain of the electrical parts differs substantially in such environments from that in more typical environments, with the result that the transducer output represents such changes, as well as the desired capacitive changes due to pressure. For example, with respect to the transducer described in U.S. Pat. No. 3,027,769, when the insulating block which separates the transducer housing and diaphragm from the film electrode is glass, its resistance changes by as much as 4 or 5 orders of magnitude when its temperature is varied from 70.degree.F to 800.degree.F. This resistance is directly in shunt with the cell capacitance to be measured and may reach a value as low as 10,000 ohms at 800.degree.F in a transducer having a total capacitive reactance of 160,000 ohms at 100 KHz. It will be recognized that such a relatively low resistance in shunt with the capacitor electrodes will significantly affect the output of the transducer, especially when the capacitance changes to be measured are relatively minute, e.g., less than 1 .times. 10.sup.-.sup.13 farad. Moisture collected on the exposed surfaces of the insulator can also substantially reduce the resistance between the electrodes provided by the insulating block. Such interference in the accuracy of a pressure measurement taken with such a transducer has inhibited their uses for many purposes for which they are otherwise ideally suited because of their ruggedness and small size.